This invention relates to ferrous alloy compositions and to methods of preparing such compositions. In a further aspect, this invention relates to dopants added to austenitic and ferritic ferrous alloys as a means of improving the elevated temperature oxidation resistance.
Compared to traditional cast iron construction, sheet metal automotive exhaust system parts such as thermal reactors and turbocharger housings would offer advantages of lighter weight and reduced thermal mass. To maximize the advantages, the metal thickness of wrought automotive engine parts should be minimized. This can be accomplished by constructing the engine parts from stainless steels, austenitic where hot strength is required, with alloying suitable for resistance to deterioration by engine exhaust gases on the inside surface of the engine parts and atmospheric air on the outside surface of the engine parts where the surface operating temperature is at a maximum. Such a construction is generally not cost effective because the resistance to oxidation of the lower cost stainless steel sheet metal alloys at elevated temperatures of 1500 degrees F. to 2200 degrees F. is not sufficient to allow their use in applications where the alloy is exposed to the combustion products normally formed by gasoline fueled internal combustion engines. Because the presently available low cost alloys do not resist oxidation in elevated temperature combustion environments, it is necessary to use a more expensive alloy with high-nickel and/or chromium content in automotive emission control devices, such as thermal reactors. Therefore, the limitation to using currently available, adequate alloy content stainless steels is the high cost and excessive strategic element content.
Degradation of stainless steels at elevated temperatures is largely dependent on the protective capacity of surface oxide films formed from the alloy during exposure to heat in oxygen containing atmospheres. This invention deals with a cost effective method of improving the protective capacity of oxide scales formed on a broad range of wrought austenitic and some ferritic stainless steels.
By way of summary, the methods of the present invention relate to the discovery that certain elements can be added to iron-base alloy materials to dramatically improve their resistance to oxidation. More particularly, the invention relates to the discovery that the addition of these elements (referred to herein as "dopants") yields lower cost materials suitable for use in heretofore impractical environments. The methods of the present invention comprise preparing an iron-base alloy composition exhibiting improved resistance to oxidation comprising the steps of:
(a) admixing in a molten state PA0 (b) allowing the admixture to cool. PA0 (a) providing an iron-containing alloy comprising PA0 (b) adding to said iron-containing alloy an effective amount of a dopant selected from the group consisting of lithium, sodium, potassium, yttrium, lanthanum, cerium, calcium, magnesium, barium, aluminum, beryllium, strontium, and mixtures thereof.
(i) an iron-containing charge; PA1 (ii) at least one alloy element selected from the group consisting of nickel, chromium, molybdenum, manganese, silicon, carbon, vanadium, cobalt, copper, nitrogen, aluminum, titanium, zirconium, and mixtures thereof; and PA1 (iii) an effective amount of a dopant selected from the group consisting of lithium, sodium, potassium, yttrium, lanthanum, cerium, calcium, magnesium, barium, aluminum, beryllium, strontium, and mixtures thereof; and PA1 (i) iron; and PA1 (ii) at least one element selected from the group consisting of nickel, chromium, molybdenum, manganese, silicon, carbon, vanadium, cobalt, copper, nitrogen, titanium, zirconium, aluminum, and mixtures thereof; and
In another embodiment, the methods of the present invention comprise the steps of:
In such an embodiment, the dopant is added to the surface of the iron-containing alloy by ion-beam surface modification, laser-induced surface modification, or by the diffusion of a surface coating.
The oxidation problems of the currently available alloy materials, such as a low-nickel austenitic (LNA) stainless steel alloys containing chromium and ferritic stainless steel alloys containing medium to high chromium can be overcome with the addition of an effective amount, preferably at least about 0.02, and more preferably about 0.1 to 2 percent by weight, of the dopants or doping elements or alloys disclosed herein.
Alloy compositions of the present invention would be made in a conventional manner, i.e., typical of the alloy without the dopant of the present invention, but with provisions for the additions of dopant elements, in the melt process or later, in the alloy processing, or by surface treatments.
In the preferred alloys, according to this invention, barium, calcium, lithium, lanthanum/cerium, magnesium, potassium and sodium or mixtures thereof are added to the alloy as dopants.
The methods disclosed herein involves the addition of small quantities of dopants (appearing for the most part in Groups IA, IIA and IIIB of the Periodic Table of Elements) to the base alloy composition. These elements, as ions, enter into the protective oxide scale and modify predominantly anion (and to a leser extent, cation) transport through the oxide scale, greatly reducing the amount of oxidation observed due to elevated temperature exposure.
Research leading to this invention was based on low nickel austenitic (LNA) alloy composition and was guided by extensive use of experimental design. Initially, a 28 run balanced orthogonal array fractional factorial scheme according to Plackett and Burman was employed as a screening method to identify main-effect influences on oxidation resistance of 26 constituents from the Periodic Table of Elements. For this work, reference was made to an article entitled "The Design Of Optimum Multifactorial Experiments" by R. L. Plackett and J. P. Burman (Biometrika, 1946, pages 305-327) which is hereby expressly incorporated by reference; an article entitled "Some Generalizations In The Multifactorial Design" by R. L. Plackett (Biometrika, 1946, pages 328-332) which is hereby expressly incorporated by reference; and to an article entitled "Table Of Percentage Points Of The T-Distribution" by Elizabeth M. Baldwin (Biometrika, 1946, page 362) which is also hereby expressly incorporated by reference. Selection criteria for elements to be considered included commercial availability in quantities sufficient to support volume alloy production, cost and subject reasoning as to the elements' ability to be a stable part of the alloy composition. Fitting these 26 constituents into the 28 run experimental design left 2 columns for random variation or error determination. The Table I lists these constituents by Periodic Table groupings. Note that La-Ce is considered as one constituent because these two elements co-exist as a commercial rare-earth product. Table II lists elements considered as part of the base composition and therefor not included in the oxidation improvement design scheme.
TABLE I ______________________________________ Periodic Table Group Constituent ______________________________________ IA Li, Na, K IIA Be, Mg, Ca, Sr, Ba IIIA B, Al IVA Si, Sn VA Pb, Sb, Bi IB Cu IIB Zn IIIB Y, La-Ce IVB Ti, Zr VB V, Nb, Ta VIB Mo, W ______________________________________
TABLE II ______________________________________ Periodic Table Group Constituent ______________________________________ IVA C VA N VIB Cr VIIB Mn VIII Fe, Co, Ni ______________________________________
Elements associated with improvements in oxidation resistance, as determined by the Plackett-Burman experimental design, were then incorporated in full factorial experimental designs of the form 2.sup.3 to 2.sup.6 for identification of interactions and 3.sup.2 to 3.sup.3 for quantifying certain effects. The notation Y.sup.X refer to X factors evaluated at Y levels each for a total of Y.sup.X test runs. Similar notations and documentation of full factorial experimental designs and analysis can be found in the literature. For this work, reference was made to "Industrial Statistics" by W. Volk (Chemical Engineering, March 1956) which is hereby expressly incorporated by reference. Interactions in this context are those situations wherein the main effect between certain variables change as a function of changes in other variables.
In the course of this research, it was found that elements within the alloy functioned in three identifiable ways: Austenite Stabilizers, Oxide Formers and Oxide Dopants. Understanding of these functions is helpful in describing the alloys of this invention.
Austenite Stabilizers
Austenitic alloys of this invention may require small compositional adjustments to maintain a stable austenitic matrix at use temperatures up to 2,220 degrees F.. Elements identified as promoting this austenite stability are Mn, Co, Ni, Cu, C, Sn, Sb, Bi and N. Throughout the course of developing this invention, it was necessary to periodically adjust the choice and quantity of austenite stabilizer elements to balance the counteracting effects of La-Ce, Ti, Zr, V, Cr, Al and Si as these elements were introduced or changed in concentration as part of the effort to determine their effect on oxidation resistance.
Oxide Formers
An object of this invention is to improve the protective nature of surface oxides formed during exposure to elevated temperatures and, therefore, a stable surface oxide is required. Elements identified as significant contributors to stable surface oxide formation on these iron base alloys are: Cr, Co, Ni, Al, and Si. The elements Cr, Co and Ni were part of the base composition, individually or in combination, Table II, and therefore not subject to elevation during the fractional factorial phase of this investigation. Al and Si were incorporated into the initial experimental design (Table I) and were determined to contribute to improved oxidation resistance through interaction with dopant elements. This interaction is interpreted to be due to the contribution of Al and Si in formation of stable surface oxides.
Dopants
Dopants are elements found to have a major effect on the protective nature of the host oxide. Typically, they are found in groups IA, IIA and IIIB of the Periodic Table of Elements and include, without limitation, those described herein, as well as mixtures of these materials. Their function in improving oxidation resistance is judged to be due to their effect on predominantly anion and to a lesser extent cation transport through the surface oxide film.